Testicular cancer is a disease that afflicts young men in their peak reproductive years. Most testicular cancers are of germ cell origin and little is known about the molecular basis of tumorigenesis in germ cells. The proto-oncogenes Ski and Sno were first identified as transforming agents in avian fibroblasts. They are widely expressed and paradoxically have been implicated in both transformation and differentiation. They act as both transcriptional activators and repressors. Preliminary data indicate that Ski and Sno are over-expressed in human testicular tumors. Ski and Sno gene expression also was detected in mouse germ cells with the highest levels observed in mitotic spermatogonia and early meiotic leptotene/zygotene spermatocytes, which span the transition between proliferation and differentiation in the germ cell lineage. Since this is the first demonstration of Ski and Sno in germ cells, their regulation and function has not been examined in germ cells These observations lead to the central hypothesis of this proposal: that altered expression of the Ski and/or Sno genes in germ cell tumors changes the nature of transcriptional regulatory complexes containing these proteins, thereby interrupting the precisely controlled proliferation necessary for normal germ cell function. The objective of this proposal is to begin to understand the function of SKI and SNO in male germ cells through two specific aims. Aim One will determine how SKI and/or SNO are altered in testicular tumors by examining gene and protein expression patterns. SKI and SNO have been implicated in several different signaling pathways, but it is unknown how they act in germ cells. Therefore, Aim Two will address this question by focusing on the protein interactions of SKI and SNO in the vitamin A deficient mouse model, in which proliferation can be controlled by the withdrawal and restoration of vitamin A. Data from this model will be used as a basis for testing the function of SKI and SNO in germ cell tumors. The data obtained from the proposed research will advance our understanding of the molecular events controlling germ cell proliferation in normal germ cells and promoting transformation in germ cell tumors.